1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Image reading apparatuses have been known that read media to be read from the upper side in the vertical direction. For example, Japanese Laid-open Patent Publication No. 2001-28671 discloses an image reading apparatus including a stand having a support arm, scanning lighting means for irradiating a surface of a document with a light beam having a slit beam shape, and document image reading means (an imaging unit) provided on an upper portion of the support arm for reading a document image by the light beam reflected from the document surface.
When an imaging unit reads a medium to be read from the upper side in the vertical direction, an optical path length between the imaging unit and a reading target position on the medium to be read changes with the reading target position (not constant). For example, when images are read while the medium to be read is being scanned, the optical path length between the imaging unit and the medium changes as the reading progresses. As a result, a depth of field corresponding to the change level is required. As a method of reducing the depth of field, the imaging unit may be provided at a high height position. In this case, however, the size of the apparatus is increased. It has been desired that suppression of increase in the size of the image reading apparatus and reduction of the depth of field are achieved together.